Normal paraffins, isoparaffins, and naphthenes, by number of rings, can be distinguished and measured in saturated hydrocarbon mixtures by a single mass spectrometric analysis. The saturates fraction can be obtained off-line by open-column or preparative liquid chromatography (LC) separations. In a preferred embodiment of this invention, this fraction is separated from the rest of the hydrocarbon mixture by LC on-line to a mass spectrometer via interfaces such as the moving belt or particle beam. Using field ionization (FI), the normal paraffins and naphthene ring types present are primarily detected as molecular ions whereas the isoparaffins are predominantly detected as characteristic fragment ions. By measuring the sensitivities of model hydrocarbon compounds representative of the various types, we are able to determine the distribution of saturate hydrocarbon types; in addition, the carbon number distribution within each type can be measured from the detected molecular ions.
Prior art determines the weight percentages of the total paraffins and naphthenes (1- to 6-rings) in a pre-separated (as discussed above) saturates fraction using high voltage (50-100 eV) electron-impact ionization (EI) mass spectrometry. "1994 Annual Book of ASTM Standards," Volume 05.02, ASTM method D2786-86. The percentage of normal paraffins is then determined by either of two methods: 1) gravimetrically by extraction of normal paraffins (molecular sieve or urea adduction) or 2) chromatographically using gas chormatography (GC). Isoparaffins are then calculated by difference between total and normal paraffins. W. L. Mead, Anal. Chem., 40, 743-747 (1968).
Field ionization has two major advantages over the prior art: a single mass spectrometric measurement provides (1) the concentrations of normal paraffins and isoparaffins as well as naphthenes and (2) the carbon number distributions within individual compound types. The pre-separated saturates fraction can be introduced by any of several inlet systems commonly employed, i.e., batch, GC, supercritical fluid chromatography, etc. The preferred embodiment of this invention offers additional advantages related to on-line separation of saturates from complex hydrocarbon mixtures: only require very small sample size, no need of time-consuming pre-separation, and enhanced chromatographic resolution. Prior art uses pre-separation and several analyses lend greater uncertainty to the accuracy of the measurements. Other advantages may include greater accuracy and selectivity.
This methodology is particularly effective for hydrocarbon mixtures boiling above 300.degree. F., preferably in the 650.degree.-1050.degree. F. range. This invention is therefore useful to characterize hydrocarbon streams where knowledge of saturates composition is important such as in catalytic cracking, lube processing, biodegradability, and diesel fuel blending, crude oil production and transport through pipeline, among other processes and products. This invention provides molecular-level characterization, by carbon number distribution, of the saturated hydrocarbon classes, thereby enabling enhanced understanding of the relationship between structure and product properties, performance, and processability in these key areas.
The distinction between and measurement of saturated hydrocarbon classes in heavy petroleum oils (with boiling points greater than 650.degree. F.) are valuable in understanding conversions and selectivities in refining processes such as catalytic cracking and lube processing. It also has value in assessing the origins of oil because several hydrocarbon types within these classes are biological markers of the oil precursors. The ability to achieve such measurements in a single analysis would improve the reliability of the measurements and facilitate process optimization using a molecular basis. In contrast, similar results from a combination of techniques applied to different sample aliquots would increase the uncertainty of the data so derived.